1. Field of the Invention
The present invention relates to a thermosensitive flow rate detecting device, and more particularly to a thermosensitive flow rate detecting device used for detecting a flow rate of intake air in, for example, an automotive engine and particularly for enhancing the detection precision for an engine pulsating flow.
2. Description of the Related Art
In general, a thermosensitive flow rate detecting device is used for metering a flow rate of air sucked to an automotive engine with a detecting circuit structure as shown in FIG. 4. FIG. 4 is a circuit diagram showing a structure of a conventional thermosensitive flow rate detecting device as disclosed in Japanese Patent Application Laid-Open No. Hei 11-351936. The structure shown in FIG. 4 will now be described.
In FIG. 4, the circuit includes a first temperature detecting resistor 3 for detecting a temperature of intake air, a heat generating resistor 4, a second temperature detecting resistor 5 disposed in the vicinity of the heat generating resistor 4 for detecting a temperature of generated heat, a power source 8, a fixed resistor 9, transistors 12a and 12b, a constant voltage power source 13 for applying a constant voltage to a bridge circuit, a flow rate detecting terminal 14, and a differential amplifier 16a. Numeral 15 denotes a flow rate output signal.
It should be noted here that the bridge circuit is formed by the first temperature detecting resistor 3 for detecting the temperature of the intake air, the second temperature detecting resistor 5 disposed in the vicinity of the heat generating resistor 4 for detecting a temperature of generated heat, and the fixed resistor 9. A constant voltage is supplied to the bridge circuit from the constant voltage source 13. An output terminal of the bridge circuit is connected to an input terminal of the differential amplifier 16a. An output of the differential amplifier 16a is connected to the heat generating resistor 4 through the transistors 12a and 12b. 
Each circuit constant of the bridge circuit is set such that the second temperature detecting resistor 5 is balanced under the condition that it is a constant temperature higher than the first temperature detecting resistor 3. Accordingly, a heating current is fed to the heat generating resistor 4 so that the input voltage difference of the above-described differential amplifier 16a becomes substantially zero. Therefore, a constant temperature difference circuit is formed in which the second temperature detecting resistor 5 and the heat generating resistor 4 are kept at a temperature that is a constant temperature higher than the temperature of the intake air.
As described above, the constant temperature difference circuit has a characteristic to follow a change in flow rate with high responsiveness because the feedback circuit is formed. For instance, in the case where the flow rate is increased, the second temperature detecting resistor 5 and the heat generating resistor 4 are cooled down, and when the resistance value is somewhat decreased, the voltage of a non-inverting input terminal of the differential amplifier 16a is increased. As a result, the output voltage of the differential amplifier is increased, and thus the emitter current of the transistors 12a and 12b is also increased. Furthermore, the heating current of the heat generating resistor 4 is increased to elevate the temperature of the heat generating resistor.
The temperature change of this heat generating resistor 4 is transferred to the second temperature detecting resistor 5 through heat conduction. Also, the temperature (resistance) of the second temperature detecting resistor 5 is elevated to the original temperature (resistance) to balance the bridge circuit.
Incidentally, during the period until the temperature of the second temperature detecting resistor 5 is returned back to the original one after the heating current of the heat generating resistor 4 is increased, there is only a heat conduction phenomenon from the heat generating resistor 4 to the second temperature detecting resistor 5, and any electrical effect does not work between the heat generating resistor 4 and the second temperature detecting resistor 5. Accordingly, when the flow rate change frequency is high, the response lag from the time the second temperature detecting resistor 5 is cooled down by the flow to decrease its temperature to the time the temperature is returned back to the original one by heating the heat generating resistor 4 becomes a problem. Also, since the alternating current property of the constant temperature difference circuit depends upon the current amplification rate of the transistors 12a and 12b, there is a problem that when the current amplification rate is changed, the responsiveness is changed.
As described above, in the prior art, there is a problem of response lag when the flow rate change frequency is high. In general, in case of the intake air of the engine, the higher the engine rpm, the higher the flow rate becomes. Accordingly, even if good response property with excellent followability is observed in the low flow rate and low rpm region, the response lag is observed for the engine pulsating flow in the high flow rate and high rpm region. As a result, there is a problem of the detection flow rate error. Also, the frequency response property is dependent upon the temperature, and in addition, the higher the temperature, the higher the current amplification rate of the transistors becomes. Thus, in the high temperature range, the responsiveness is high and more resonant, whereas in the low temperature range, the responsiveness is attenuated, and depending upon the situation, there is the response lag resulting in a detection error as described above.
Also, in the case of the power source change, the power source change tends to appear to be imposed with the output signal through the heating current feeding transistor 12a. When the power source change becomes remarkable, there is a problem that the detection flow rate error would occur.
In order to overcome the above-described problems, an object of the present invention is to provide a thermosensitive flow rate detecting device in which no response lag is caused even if the flow rate change frequency is high, and no error in detection flow rate occurs even if the power source change would occur.
In order to attain this and other objects, according to the present invention, there is provided a thermosensitive flow rate detecting device comprising: a heat generating resistor provided in a fluid to be measured for generating heat by electric power consumed in accordance with a flow rate of the fluid to be measured; a first temperature detecting resistor for detecting a temperature of the fluid to be measured which changes in accordance with the flow rate; and a second temperature detecting resistor for detecting the temperature of the heat generating resistor, further comprising a bridge circuit provided with the first temperature detecting resistor and the second temperature detecting resistor, for controlling heating current of the heat generating resistor so that a temperature difference between the first temperature detecting resistor and the second temperature detecting resistor is kept constant, and for detecting the flow rate within the fluid to be measured by using the heating current, wherein a voltage in proportion to the heating current of the heat generating resistor is applied to the bridge circuit.